The invention relates to a method for the continuous manufacture of elastomeric compounds. More particularly the invention relates to the continuous manufacture or mixing of rubber and rubber related compounds utilizing an extruder.
The predominant prior art method of mixing rubber is the use of internal mixers, such as the Banbury.RTM. mixer manufactured by Farrel Corp. Internal mixers are used in batch type operations in which an elastomeric compound and other ingredients of the mixture are added at various times.
Internal mixers of the Banbury.RTM. type, because of dusting and mixing problems, often require the addition of ingredients in small increments. This requires, in most cases, the presence of workers in the mixer area at all times. Also, because such mixers are substantially open, fumes from the mixture often permeate the atmosphere in the area of the mixer.
Because of the various problems with internal mixers, and because of the chemical nature of some mixtures, in some cases not all the ingredients of the mixture can be mixed together at one time. As a result, an elastomer may be mixed with some of the ingredients, and then discharged or dumped and held in inventory as an intermediate product. The intermediate product can be further processed when needed, and additional ingredients may be added and mixed therein.
Each mixing step in the internal mixer is known in the art as a "pass".
While some mixtures require only a single pass, others may require as many as four or five passes through conventional mixers. This is time consuming and labor and capital intensive. Furthermore, when using conventional batch mixing devices, the operator must observe stringent quality control procedures that help maintain uniformity from one batch to another.
Accordingly, the industry has attempted to develop a process by which rubber can be produced in a single pass or in a continuous process. One such attempt is a machine which is essentially an internal mixer having a screw conveyor or an extruder located below the rotors of the mixer. See, for example, GB 2 173 441; GB 2 191 713; and GB 1 550 364.
For a discussion of the slow adoption of continuous compounding in the rubber industry, see "A Tale of Continuous Development", European Rubber Journal, by Harry Ellwood, March 1987.
Twin screw extruders are commonly used in the plastics industry to prepare plastic compositions, in particular thermoplastic compositions. Plastics tend to become less viscous at higher temperatures (prior art methods usually include heating the plastic in the extruder to bring the plastic to its melt temperature), and the wetting action of plastics at processing temperatures improves the processability of the plastic in a twin screw extruder.
Elastomers typically do not show a sharp melting point and, at normal processing temperatures, are very viscous, and may tend to either cure or degrade during mixing because of the heat caused by the friction of mixing. If mixed in a twin screw extruder in the same manner as plastics, the elastomer resistance and friction would tend to damage the compound and may even result in damage to the extruder.
It is an object of the present invention to provide a method for mixing rubber in a continuous process and to provide a method for mixing rubber wherein improved dispersion of the added ingredients is obtained.
Other objects of the invention will be apparent from the following description and claims.